1. Field of the Invention
The present invention relates to an aligning apparatus for mutually aligning a first substrate with a second substrate, and more particularly to an aligning apparatus adapted for use, for example, in an exposure apparatus for semiconductor device manufacture.
2. Related Background Art
For use for example in an exposure apparatus, the U.S. Pat. No. 4,251,160 discloses an aligning apparatus utilizing grating marks both on a mask constituting a first substrate, and a wafer constituting a second substrate. Also there is known an aligning apparatus in which the mask and the wafer have grating marks of mutually different pitches, and the alignment of the mask and wafer is achieved by detecting the position of a moire fringe pattern, generated by optically superposing said grating marks, with an image sensor. Though such known apparatus can detect a very small displacement with a high precision, fetching of the image information of the moire fringe pattern requires a certain time due to the limitation in the clock frequency of the image sensor, so that, in case of an aligning operation of an object in motion, the precision of positional measurement is deteriorated due to the movement of said object during said time.